What is life?

Properties of Living Things

     Life is hard to define, and many things share some but not all characteristics of life. There is not one, fully accepted definition of a living thing. A widely used definition states that to be alive something must have the seven following characteristics:

  1. Maintain homeostasis: maintenance of internal conditions that are separate from environment.
  2. Have organization: hierarchal non-random structure.
  3. Have metabolism: transformation of energy into usable form. Building (anabolism) and decomposing (catabolism) organic matter.
  4. Be adaptable: the ability to change and evolve over time as a function of environment.
  5. Respond to stimuli: This can take various forms, from feeling pain when touched to increasing energy capture under favorable conditions.
  6. Reproduce: The ability to produce progeny, either asexually or sexually.
  7. Grow: Increase in size of the whole and its parts.

These characteristics apply to life on Earth but also, potentially, to life we could find on other planets or other solar systems.    

Features of Earth Life

Life on Earth has some shared features that derive from our shared evolutionary heritage. All Earth life has:

  1. Cells (one or many)
  2. Lipid membranes
  3. DNA to RNA to protein information flow
  4. Ribosomes